One of the most common mistakes programmers do is to treat the DateTime as string, or in another words always assuming a specific date format.

How DateTime Works
In strong typed languages like Java and .Net, DateTime contains the
date and time value as a consolidated numeric value without any format specification.
The culture aware ToString() method of DateTime converts the time value
according to the culture/format specifications of the system and configuration.

How to use?
Always use DateTime type for date and time values especially when savin/retriving DateTime values from/to Database.
Always use command parameters with proper types for passing values from application to database.

What to avoid?
Using string to contain DateTime values can become a disaster especially in large application.
You may find yourself trapped in a Format-Hell if you are assuming any particular format in calculation or representation
Never pass Datetime values as string across platforms or contexts for instance application-to-database